Self-scanned photodiode array with selectively-skipped pixels

ABSTRACT

A self scanning photodiode array allows low signal pixels to accumulate charge for multiples of predetermined exposure time t 0  before being read. The pattern of exposures, the integers M i  where I runs from 1 to N the number of pixels in the array, is chosen such that the pixels of interest accumulate as much charge as possible without exceeding saturation.

CROSS REFERENCE RELATED APPLICATION INFORMATION

This application is a reissue application of U.S. patent applicationSer. No. 10/598,614 filed Mar. 30, 2007, issued as U.S. Pat. No.7,795,570 on Sep. 14, 2010, which is the U.S. National Stage ofInternational Application No. PCT/US05/06671 filed Mar. 2, 2005, whichclaims priority from U.S. Provisional Patent Application No. 60/552,056,filed Mar. 10, 2004. The contents of these applications are incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to spectro-photometers adapted to be used as amulti-purpose spectro-photometer or a detector for analysis. Inparticular, this invention relates to an improvement in thesignal-to-noise ratio of measurements made using a self-scannedphotodiode array to detect light in the UV, visible and IR portions ofthe spectrum.

BACKGROUND OF THE INVENTION

In the application described here, light containing a range ofwavelengths is dispersed and focused onto a photodiode array so that theoutput of the array is the spectrum. Typically the amount of lightvaries strongly with wavelength so that the pixels of the array generatephoto charge at widely disparate rates.

A scan of the array is also made with the light blocked by a shutter todetermine the dark current contribution and any offset due to theamplifying electronics. These dark values are subtracted to produce Ndigital values which measure the photo charge accumulated on each of theN pixels in time t₀. A number of such scans are normally co-added sothat an averaged set of dark-corrected pixel signals is created at somepredetermined sample interval, or sample time.

Assuming that the light intensity falling on the diode array does notfluctuate, there are two sources of noise which degrade the precision ofmeasuring the charge accumulated on each pixel. First there is shotnoise, proportional to the square root of the accumulated charge. Thisis an unavoidable consequence of the discrete nature of electron flow.If this were the only noise source, the signal-to-noise ratio wouldimprove as the square root of the amount of charge accumulated each timea pixel is read, and also as the square root of the number of times thepixel is read. Therefore, the signal-to-noise ratio would not depend onthe exposure time t₀, only on the rate of charge generation by thelight. However, the second source of noise, so-called fixed noise, isindependent of the light intensity, and corrupts the signal every timethe pixel charge is read. It has two components: read noise, otherwiseknown as kTC noise, originating in the PDA device, and noise from theexternal amplifier. There is also a shot noise contribution from thepixel dark current, but at the relatively high light levels consideredhere it can be ignored.

If the light signal is high enough, shot noise dominates, which isproportional to the square root of the signal. But if the signal falls,the fixed noise dominates the signal-to-noise ratio, which now degradesin direct proportion to the falling signal.

In prior art methods it is standard practice to select the longest timeperiod possible for t₀ with the condition that none of the pixelssaturate within the spectral range of interest for a particularexperiment or series of measurements. The exposure time t₀ isestablished by measuring the full spectrum with a reference material inthe sample cell, using a short exposure time where it is known that nopixels will saturate. The exposure time t₀ is then found by scaling theshort exposure time so that the highest-signal pixel within the spectralrange of interest will be, for example, 85% of saturation.

In the case of HPLC detection, the reference material is mobile phase atthe start of a separation when no absorbing sample is present. Selectingt₀ as long as possible minimizes the number of reads of the array withinthe sample time, thereby minimizing the effect of read noise added tothe pixel signals. Pixels outside the spectral range of interest maysaturate. The self-scanning process reads all of the pixels in the arrayat intervals of t₀, and the values for those pixels outside the range ofinterest are simply ignored. Some of those pixels may saturate, but thisdoes not affect data from pixels within the desired spectral range.

To optimize data collection and signal-to-noise ratio, exposure time t₀must be a sub-multiple of the sample time, which is the reciprocal ofthe data rate (i.e. the number of times per second the pixel values arereported). This condition assures that data collection is continuous,with no waiting periods when measurement time is lost.

Prior art approaches select the best exposure time t₀ for the pixelswith the highest signals, and the signal-to-noise ratio is optimized forthese pixels. However other pixels in the array may receive much loweramounts of light and accumulate charge at a much lower rate. Thesepixels are read out with a large signal-to-noise disadvantage since thefixed noise associated with each read is constant, while the accumulatedcharge signal is low.

SUMMARY OF INVENTION

The inventive method and apparatus improve the signal-to-noise ratio ofmeasurements made using a self-scanned photodiode array to detect lightin the UV, visible and IR portions of the spectrum. According to theinvention, a linear array of N pixels receives light creating electronsand holes in the pixel junction region. Charge is accumulated m thepixel and associated capacitor at a rate proportional to the lightintensity and quantum efficiency of the photodiode. In prior artdevices, after an exposure time t₀, the self-scanning process isinitiated and the charge residing in each pixel is switched sequentiallyonto the output video line. Each charge packet is amplified, passed to asample-and-hold circuit, converted to a digital value and passed to adata system.

Advantageously, the inventive method allows low signal pixels toaccumulate charge for multiples of the exposure time t₀ before beingread. For example, the ith pixel is exposed for M_(i) read cycles, or atime of M_(i)t₀. This is accomplished by disabling or skipping the readsignal to the ith pixel for (M_(i)−1) read cycles, reading it everyM_(i)th scan of the array. The pattern of exposures, the integers M_(i)where i runs from 1 to N the number of pixels in the array, is chosensuch that the pixels of interest accumulate as much charge as possiblewithout exceeding, for example, 85% of saturation.

In order to report data from the array at a chosen sample rate (forexample 10 points or spectral scans per second), permitted exposurevalues M_(i)t₀ are restricted to sub-multiples of the sample interval,in this case 100 ms. In a first illustrative embodiment, if t₀ were setat 12.5 ms, permitted values for the integers Mi would be 1, 2, 4 and 8,yielding pixel exposures of 12.5 ms, 25 ms, 50 ms and 100 ms.

It is contemplated within the scope of the invention that more exposurechoices are possible if the self-scanning clock speed is higher thanneeded for the shortest exposure t₀, or if the sample interval islonger. In a second illustrative embodiment, if it is desired to obtaina reading of the array once every 300 ms and the clock speed allows thearray to be scanned every 6.25 ms, then permitted exposures in ins are:6.25, 12.5, 18.75, 25, 37.5, 50, 75, 100, 150, 300, with correspondingvalues M_(i) of: 1, 2, 3, 4, 6, 8, 12, 16, 24, 48.

It may be that the shortest exposure used for any pixel is still 12.5ms, and the longest 100 ms, but a change to the clock speed allows 7exposure choices versus 4 for the first example. More choice meansoverall better signal-to-noise optimization of the individual pixelexposure times M_(i)t₀.

As noted above in discussing prior art devices, the dark signal madewith a shutter closed is subtracted from each pixel signal to determinethe digital value attributable to the light falling on that pixel. Inthe case of the present embodiment, the dark signal must of course bemeasured using the same exposure pattern of integers M_(i).

When the self scanned array according to an embodiment of the presentinvention is read, N charge packets are measured every scan. Thoserelating to skipped pixel reads have a value which is nominally zero.However they are not exactly zero because read noise and amplifier noiseare present. The data system sets these values to zero exactly, so thatwhen values from the several scans of the array are co-added and thespectrum is reported at the end of the sample time, only noise is addedwhen pixels are actually read. In this way the fixed noise is minimized.In an instrument using the inventive method, shot noise will dominateover the whole array, and fixed noise will not be a factor. Ininstruments using prior art photodiode arrays, the signal-to-noise ratioof pixels with low signals is dominated by fixed noise, which may beseveral times greater than the shot noise.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present inventionwill be more fully understood from the following detailed description ofillustrative embodiments, taken in conjunction with the accompanyingdrawing in which:

FIG. 1 is a schematic representation of a self scanning photo-diodeaccording to the invention;

FIG. 2 is a graphic depiction of the self scanning process according tothe invention that follows the clock signal until all diodes have beenread;

FIGS. 3A and 3B are graphic illustrations of spectra illustrating deviceoperation; and

FIG. 4 is a flow diagram of a method according to the invention forimproving signal to noise ratio according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Detailed embodiments of the present invention are disclosed herein,however, it is to be understood that the disclosed embodiments aremerely exemplary of the invention, which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

An apparatus and method in accordance with the present invention allowsthe use of a self scanned photo-diode array to detect light in the UV,visible and IR portions of the spectrum with greater sensitivity.Turning to FIG. 1, a block diagram depicts the device according to theinvention. As shown in FIG. 1, timing information 102 is supplied in theform of electrical signals to the device. A photo diode array 104according to the invention is comprised of individual pixels(photo-diodes). In a first illustrative embodiment the photo diode array104 is comprised of 512 pixels that are represented by capacitorslabeled 1 to 512. The photodiode array is constructed as a semiconductordevice in a manner known in the art of conventional semiconductor deviceand photo diode array technology. It is contemplated within the scope ofthe invention that photo diode array technology such as implemented in aself-scanned photodiode array, part number PI 0512 WSN, available fromPeripheral Imaging Corporation, San Jose, Calif., or the like may beused. The pixels within the photo diode array 104 accumulate charge at arate in proportion to the optical power falling on them. Charge can bemeasured in pico coulombs, pC. The full well capacity of the pixels inthis first illustrative embodiment is about 60 pC. The rate of chargeaccumulation can be expressed as a photocurrent in nano amps, nA.

The conventional self-scanned photo-diode array lacks an Enabling SwitchControl 108, which is implemented according to the invention. The pixelsare first reset, and charge accumulates according to the amount ofincident light and the pixel response, which varies with wavelength.After a predetermined exposure time, a start pulse that is suppliedexternally initiates the self scanning process, which follows the clocksignal until all pixels (photodiodes) within the photo diode array 104have been read as depicted in FIG. 2. In this first illustrativeembodiment, the next rising edge of the clock signal after the startpulse causes charge to be switched from a first pixel 106 (photo diode)to an output video line 110, where it passes to a charge amplifier, A/Dconverter and then to the data system. The first pixel 106 (photodiode)is reset and the next clock rising edge causes charge from second pixel112 (photodiode) to be switched to the output video line 110. Thisprocess continues until pixels (photodiodes) within the photodiode array104 have been read out and reset. A typical clock frequency is about 100kHz. This frequency would cause the complete scanning process to takeabout 5 ms for a photodiode array 104 having 512 pixels.

As explained above, the best signal-to-noise ratio results when theexposure time is as long as possible so that the noise associated witheach read cycle is minimized. In conventional devices all pixels(photodiodes) are read with the same exposure time, even if some of thepixels (photodiodes) have only accumulated a fraction of their chargecapacity.

According to the invention, the Enabling Switch Control 108 is used todetermine whether a particular pixel (photodiode) is read out after oneexposure time t₀, or whether a Self-scanning Switch Control 114 signalto that pixel is blocked until its charge accumulation is optimum. Thatis, the enabling switch control 108 enables or disables pixel scanningas done by the self-scanning switch control 114.

In an illustrative embodiment shown in FIG. 2, a number of pixels areshown which generate photocurrent in a descending order from left toright. The timing pulses put out by the Enabling Switch Control 108 areshown below for the first 8 read cycles. In this illustrativeembodiment, a high value of the enabling signal allows the self scanningprocess to take place and a low value prevents a pixel from being read.

Turning to FIG. 2, a first self-scanned read cycle is initiated at timet₀ 202, which in this illustrative embodiment is approximately 12.5milliseconds from the start. A second read cycle is initiated at time2t₀ 204, which in this illustrative embodiment is approximately 25milliseconds from the start. A third read cycle is depicted at 3t₀ 205,and so on to the eight read cycle initiated at 8t₀ 208, which isapproximately 100 milliseconds from the start.

Pixels (photodiodes) with highest-signals 210 are read out every readcycle. The highest signal pixel 210 produces a charge close to the fullwell capacity, but below saturation 220. The first group of pixels 210,which includes the pixel with highest signal, produce photocurrentsequal to half or more of the highest signal pixel. A second grouping ofpixels are medium signal pixels 212 that produce photocurrents which areless than 50%, but more than 25% of the highest-signal pixel 210photocurrent. These acquire more optimum charge if they are allowed toexpose for two exposure cycles 2t₀ 204, shown at the top of FIG. 2. Thisis accomplished by an Enabling Switch Control signal shown at time 2t₀204.

The photocurrents of pixels in a third grouping are lower signal pixels214 that produce between one quarter and one eighth of thehighest-signal pixel 210, and these are read at time 4t₀ 206. Thephotocurrents of pixels in a fourth grouping are lowest signal pixels216 that produce signals below one eighth of the highest-signal pixel210 and are read at time 8t₀ 208. At this point all the pixels ofinterest have been read, and the whole sequence repeats.

Data system software accumulates the readings from each pixel toreconstruct the spectrum at the end of each 8t₀ 208 interval. Theself-scanning process produces a value every clock cycle, whether or notthe switch to a particular pixel is enabled. The system software ensuresthat zero is recorded when a pixel is not read, so that read noise forthese cycles is not added to the result.

Spectra illustrating device operation according to the invention areshown in FIGS. 3A and 3B. FIG. 3A shows a portion of a deuterium arcspectrum from 200 to 500 nm, dispersed across a portion of aself-scanned photodiode array. In recording this spectrum, the EnablingSwitch Control was enabled at all times. Each point on the plot is thesignal from one pixel. Each pixel is read every read cycle and theresult is the same as if a conventional self-scanning photodiode hadbeen used. FIG. 3B shows pixel signals when the Enabling Switch Controlis operated according to the invention as described in the context ofFIG. 2. Different geometric shapes show exposure times for differentpixels. As shown in FIG. 3B, an exposure time of 12.5 ms 302 isrepresented by diamonds; an exposure time of 25 ms 304 is represented bysquares; and exposure time of 50 ms 306 is represented by triangles; andan exposure time of 100 ms 308 is represented by circles. All pixelshave optimized exposure times, minimizing the number of read cycles andenhancing the signal-to-noise ratio.

A method according to the invention for improving signal to noise ratiois depicted in FIG. 4. The method of improving signal to noise ratio ofmeasurements is made using a self-scanned photodiode array enhanced withthe addition of an Enabling Switch Control, as described hereinbefore,to detect light in the ultraviolet, visible and infrared portions of alight spectrum.

At time zero all the pixels are reset and start to accumulate charge.The pixels are exposed to light corresponding to a reference conditionsuch as a sample cell filled with solvent, but with no analyte. Allpixels are exposed to a short exposure time in which no pixels aresaturated and read 400. The shutter is closed, and a dark spectrum ismeasured using the same short exposure time and recorded 402. The darkspectrum is subtracted and a dark-corrected reference spectrum iscalculated 404. These spectra are energy spectra, plotting photocurrentmeasured in A/D counts versus wavelength.

Frequently, a sample rate will be specified which is related to thewidth in time of the analyte peaks passing through the flowcell. Forexample, a sample rate of 10 spectra per second means that a spectrum ofthe contents of the sample cell will be output every 100 ms (T₀). A unitof exposure time t₀, is selected which is longer than the time needed toread the full array, and shorter than the time for any pixel in theregion of interest to saturate t₀ must be an integer sub-multiple of T₀.According to the invention, allowed exposure times are integermultiples, Mt₀, of the unit of exposure time t₀, which are alsosub-multiples of T₀. It is advantageous to choose a value of t₀ suchthat the maximum value of M=T₀/t₀ has many factors. The more allowedchoices of M, the fewer read cycles overall and the more closelyoptimized the signal-to-noise ratio of the pixel signals.

Using the example given in FIG. 2, if T₀ is 100 ms, and the array can beread in less than 10 ms, a set of allowed exposure times would be 12.5,25, 50 and 100 ms. t₀ is 12.5 ms, giving integer values M=1, 2, 4 and 8.Note that integers 3, 5, 6 and 7 are not allowed values of M, becausethey do not divide into T₀/t₀.

The next step is to assign values of M to every pixel in the wavelengthrange of interest. A dark-corrected reference spectrum is used todetermine the exposure time which each pixel would need individually toreach a predetermined level such as 85% of saturation and such level iscalculated 406. Using sample rate and array read time unit of exposuretime t₀ is determined 408. The largest values of M from the above listto each pixel are assigned 410 such that the calculated exposure timeneeded for a given pixel to reach 85% saturation is equal to or largerthan the exposure time associated with M. For example set M₅₀ =2 if the50^(th) pixel would reach 85% of saturation in 20 ms; set M₂₅₀=8 if the250^(th) pixel would reach 85% of saturation in 65 ms, and so on.Depending on the range of pixel signals in a particular situation, theremay be some values of M which are not assigned to any pixel.

The reference spectra is re-measured 412 using an exposure patternspecified by assigned integers M_(i). The dark spectrum is alsore-measured 414 using the exposure pattern specified by the assignedintegers M_(i). Using the re-measured spectra, specified by the assignedintegers M_(i), the dark-corrected reference spectrum is calculated 416.Sample spectra is than measured 418 using the same exposure patternspecified by the assigned integers, which is repeated each sample timeT₀, and dark-corrected sample spectra is calculated. The reference andsample spectra are combined and the absorbance spectrum is calculated420 as follows; A absorbance units, over the range of wavelengths ofinterest, by combining dark-corrected sample and reference spectra:A=log₁₀(reference/sample).

Although the instant invention is described using a photodiode arraycontaining 512 individual pixels (photodiodes) it should be appreciatedby those skilled in the art that the inventive method can be utilizedwith photodiode arrays having less or more individual pixels(photodiodes).

Although the instant invention is described using selected criteria forpixel saturation levels, it should be appreciated by those skilled inthe art that variable saturation levels can be assigned to individualpixels or groups of pixels.

Various other changes, omissions and additions in the form and detail ofthe present invention may be made therein without departing from thespirit and scope of the invention. Therefore, the above descriptionshould not be construed as limiting, but merely as exemplification ofthe various embodiments.

What is claimed is:
 1. A device having a self-scanned photodiode arraywherein charge from individual pixels is switched from each pixelsequentially onto at least one output video line after a predeterminedexposure time (t₀) comprising: at least one light source; at least onesample cell having means for receiving light from said at least onelight source; a photo-diode array having pixels for collecting lighttransmitted through said at least one sample cell, wherein the intensityof said light may vary across the spatial extent of the array; means forreading said pixels; means for skipping the reading of selected pixelsfor one or more additional exposure times t₀ allowing said selectedpixels to be exposed for specified integer multiples (M) of saidpredetermined exposure time t₀, thereby allowing said selected pixelsreceiving less light to accumulate additional charge before being readout and thereby reducing the number of read cycles and improving thesignal-to-noise ratio.
 2. The self-scanned photodiode array of claim 1wherein different pixels are exposed for the same or different integermultiples (M) of said predetermined exposure time.
 3. The self-scannedphotodiode array of claim 1 in which said predetermined exposure time t₀does not exceed saturation of the pixel or pixels accumulating charge ata highest rate within a predetermined range of pixels.
 4. Theself-scanned photodiode array of claim 1 wherein specified integermultiples M are chosen such that each pixel signal, within apredetermined range of pixels approaches but does not exceed saturation.5. The self-scanned photodiode array of claim 1 wherein said exposuretime of individual pixels, Mt₀, does not cause saturation of pixels fromwhich charge is measured.
 6. The self-scanned photodiode array of claim1 wherein said exposure time of each pixel, is an integer multiple M ofsaid predetermined exposure time t₀, wherein the lowest value of M isgreater than one.
 7. The self-scanned photodiode array of claim 1wherein a sample time is defined as a time taken for one or morecomplete measurements of the full or selected portion of the photodiodearray, said individual pixel exposure times Mt₀ being submultiples ofsaid sample time.
 8. The self-scanned photodiode array of claim 1wherein said predetermined exposure time t₀ is established when saidphoto-diode array receives said light according to a referencecondition.
 9. The self-scanned photodiode array of claim 3 wherein thepredetermined range of pixels includes the full array.
 10. Theself-scanned photodiode array of claim 4 wherein the predetermined rangeof pixels includes the full array.
 11. The self-scanned photodiode arrayof claim 1 wherein the value recorded when a pixel read is skipped isset to zero to avoid the addition of unnecessary read noise.
 12. Amethod of improving signal to noise ratio of measurements made using aself-scanned photodiode array to detect light in the ultraviolet,visible and infrared portions of a light spectrum comprising the stepsof: exposing pixels of said photodiode array to light received from areference condition and measuring the spectrum of a short exposure timecausing no saturation of said pixels; measuring a dark spectrum of saidpixels with shutter closed using said short exposure time; calculating adark corrected reference signal from each pixel; calculating an exposuretime for each pixel such that its accumulated charge would reach apredetermined level, close to but below saturation; establishing apredetermined exposure time t₀, short enough that no pixel in aspecified range of interest will saturate; assigning integers M suchthat individual pixels are exposed for integer multiples of thepredetermined exposure time Mt₀, such that after time Mt₀ saidindividual pixels have accumulated charge close to but not exceedingsaturation; re-measuring dark spectrum with shutter closed usingexposure pattern determined by integers; re-measuring reference spectrumusing exposure pattern determined by integers, thereby creating adark-corrected reference spectrum; Measuring sample spectra using thesame exposure pattern determined by integers and creating dark correctedsample spectra; combining reference and sample spectra to determineabsorption characteristics of sample and thereby identify and quantitatesame with improved signal-to-noise ratio.
 13. A self-scanned photodiodearray wherein charge from individual pixels is switched from each pixelsequentially onto at least one output video line after a predeterminedexposure time (t₀) comprising: an array of photodiode pixels forcollecting light, wherein the intensity of said light may vary acrossthe spatial extent of the array; means for reading said pixels; meansfor skipping the reading of selected pixels for one or more additionalexposure times t₀ allowing said selected pixels to be exposed forspecified integer multiples (M) of said predetermined exposure time t₀,thereby allowing said selected pixels receiving less light to accumulateadditional charge before being read out and thereby reducing the numberof read cycles and improving the signal-to-noise ratio of the measuredlight.
 14. A method of improving signal-to-noise ratio of measurementsmade using a photo-diode array including a plurality of pixels, themethod comprising: exposing, for a first reference exposure time, theplurality of pixels of the photo-diode array to light received from areference light source; measuring a first reference spectrum of thelight received by the plurality of pixels during the first referenceexposure time, the length of the first reference exposure time selectedsuch that no saturation occurs in the plurality of pixels; closing ashutter of the photo-diode array for a second reference exposure timeequal in length to the first reference exposure time; measuring a firstdark spectrum of the plurality of pixels during the second referenceexposure time; calculating, using the first reference spectrum and thefirst dark spectrum, a first dark-corrected reference signal from eachpixel from the plurality of pixels; calculating an exposure time foreach pixel from the plurality of pixels such that accumulated chargewould reach a predetermined level below saturation; determining a unitexposure time t₀ that is less than each of the exposure times associatedwith the plurality of pixels; assigning integer multiples of M to eachpixel from the plurality of pixels such that, for each pixel, the timeequal to M integer multiples of the unit exposure time t₀ is less thanthe exposure time for the particular pixel; and measuring a second darkspectrum for each pixel from the plurality of pixels with the shutterclosed, wherein the second dark spectrum for each pixel is measured fora time period equal to the unit exposure time t₀ times the assignedintegar M assigned to the pixel being measured.
 15. The method of claim14 further comprising: measuring a second reference spectrum of thelight received by the plurality of pixels from the reference lightsource, wherein the second reference spectrum for each pixel is measuredfor an exposure time period equal to the unit exposure time t₀ times theassigned integer M assigned to the pixel being measured; calculating,using the second reference spectrum and the second dark spectrum, asecond dark-corrected reference signal from each pixel from theplurality of pixels; exposing the plurality of pixels of the photo-diodearray to light received from a sample source; measuring a samplespectrum of the light received by the plurality of pixels from thesample source, wherein the sample spectrum for each pixel is measuredfor an exposure time period equal to the unit exposure time t₀ times theassigned integer M assigned to the pixel being measured; and determiningabsorption characteristics of the sample source using the samplespectrum and the second dark-corrected reference signal.